1. Field of the Invention
This invention relates to a dielectric imaging member and processes for producing an image on a dielectric imaging member, such as polyester film.
2. Description of the Prior Art
Images can be reproduced on a dielectric surface through various electrostatographic processes. In one such process, an electrostatic latent image is generated on the dielectric from a metallic electrode or pin by air ionization. Other such processes involve the transfer of an electrostatic latent image to a dielectric surface after it has been formed either on a dielectric surface or on a photoconductive surface.
In the first such process, generally referred to as electrography, electrostatic latent images are generated by character shaped electrodes or pin electrodes which are brought into close proximity to an insulating surface, such as a dielectric web, supported on a base electrode. A potential is applied across the electrodes below a critical stress value. Transfer of the character or pin configuration from the electrode to the insulating web is effected by the use of a relatively low potential triggering pulse which raises the electric field above the critical stress value to produce a field discharge in the space between the insulating web and the electrode. The discharge action gives rise to the formation of an electrostatic latent image of the character or pin on the insultating web. Thereafter, the generated image on the insulating web can be rendered visible by application of liquid or dry developer thereto.
Electrography is useful in many applications where it is required that a voltage signal pulse be applied directly to a dielectric receiving member, e.g., analog oscillographs, high speed line printers, digital plotters and the like. Typical requirements for such systems are pulses of 700 volts in 50 to 100 microseconds.
It has been found, however, that application of electrographic techniques to dielectric films of finite thickness, e.g., polyester films having a thickness of greater than 75 micrometers, results in a low density, diffuse image that requires relatively high voltage pulses in the millisecond range.
In the basic electrostatographic process, a uniform electrostatic charge is deposited upon a photoconductive insulating layer which is thereafter exposed to a light image to selectively dissipate charge in those areas of the layer exposed to the light, thereby forming an electrostatic latent image. The image can be developed by depositing a viewable toner thereon. Alternatively, the latent image can be transferred to a dielectric surface for subsequent development. Still further, the latent image formed upon dielectric surface either by electrographic techniques or transfer techniques as described above can, in turn, be transferred to a dielectric surface. Techniques for transferring electrostatic latent images to an insulating surface are well known in the art and have been accorded the acronym TESI (Transfer of ElectroStatic Image) -- see, for example, Xerography And Related Processes, Dessauer and Clark, The Focal Press (1965) pp. 405 et seq.
If desired, the latent image on the photoconductor can be developed directly and the developed areas can then be transferred by placing an insulating surface over the developed photoreceptor surface and applying thereto a high potential of opposite polarity to that of the developer by, for example, corona discharge. When the insulating surface is peeled away from the photoreceptor, it will carry thereon a sizeable portion of the developer in image configuration. When transfer is accomplished, the developer can be fixed by fusing it to the receiving surface or by other conventional fixing means.
When the electrostatic latent image is transferred from the photoconductor surface to a dielectric surface before development, the charged photoconductive surface is brought into intimate contact with the dielectric surface to effect charge transfer or transfer can occur across an air gap by impressing a voltage between a conductive backing on the photoconductor and the opposed dielectric surface of a polarity to attract the charge on the imaged areas to the dielectric.
As a practical matter, however, it has heretofore been impossible to transfer charge from a photoconductor to a thick e.g., 75 micrometers or greater, dielectric film and subsequently develop the image to a high optical density with conventional systems.
Accordingly, it is an object of the present invention to provide an improved dielectric film which can be readily imaged using various electrostatographic techniques to provide high density, sharp images.
It is another object of the present invention to provide a dielectric film having a thickness greater than about 75 micrometers which can be electrographically imaged and result in the formation of high optical density, sharp images.
It is a still further object to provide improved micro-imaging systems.
These as well as other objects are accomplished by the improved dielectric member of the present invention which comprises a dielectric substrate having a thickness between about 75 and 175 micrometers, a conductive coating on said substrate and a dielectric material having a thickness less than about 15 micrometers overcoating said conductive coating.
Transparent polyester films are presently used as the preferred base material, for example, in micrographics, due to their strength and stability. To permit process manipulation and handling, these polyester films are typically about 75-175 micrometers thick. Accordingly, reproduction processes based on the generation of an electrostatic latent image on such dielectric substrates were heretofore unavailable for use in conjunction with such preferred, but relatively thick substrates.